A mechanical power recovery type refrigeration cycle apparatus has been known conventionally in which an expander recovers the energy of expanding working fluid and the recovered energy is used as a part of the power for driving a compressor (see, for example, JP 2001-116371 A).
As one type of expander, a rotary expander has been known. The rotary expander includes a cylinder and a piston that performs an eccentric rotational motion in the cylinder, and a working chamber that changes its internal volumetric capacity according to the eccentric rotational motion of the piston is formed between the cylinder and the piston. In the rotary expander, the following processes are carried out in sequence by the eccentric rotational motion of the piston: a suction process in which a working fluid is drawn into the working chamber through a suction port; an expansion process in which the working fluid expands in the working chamber; and a discharge process in which the working fluid is discharged through a discharge port. In the suction process, the volumetric capacity of the working chamber increases while the suction port is in communication with the working chamber. In the expansion process, the volumetric capacity of the working chamber increases while the suction port and discharge port are not in communication with the working chamber. In the discharge process, the volumetric capacity of the working chamber decreases while the working chamber is in communication with the discharge port.
In the case of what is called a single-stage rotary expander having only one cylinder, the suction process, expansion process and discharge process must be completed during one rotation of the piston in the cylinder. During the processes, the rate of the working fluid flowing into the working chamber increases gradually according to the rotation of the piston in the cylinder after the suction port opens, and then decreases and becomes zero at the end of the suction process. Accordingly, rapid fluctuation of pressure of the working fluid, which is called “pulsation”, occurs in the suction port.
In view of this, a two-stage rotary expander having two cylinder-piston pairs has been proposed (see, for example, JP 2005-106046 A). The two-stage rotary expander disclosed in JP 2005-106046 A includes a first cylinder and a second cylinder. A working chamber on the downstream side in the first cylinder and a working chamber on the upstream side in the second cylinder are connected to each other via a communication passage. The suction process, expansion process and discharge process of the working fluid are carried out in the first cylinder, communication passage and second cylinder in an integrated manner. According to the description of JP 2005-106046 A, in this two-stage rotary expander, the rate of the working fluid flowing into the working chamber increases gradually according to the rotation of the piston in the first cylinder after the suction port opens, and then decreases gradually to zero. Therefore, it has been conceived that a rapid change in the inflow rate of the working fluid is suppressed and thus the pulsation of the working fluid can be suppressed.
The present inventors, however, have found, as a result of intensive studies, that even in this type of two-stage rotary expander, pulsation of the working fluid still occurs in association with the drawing thereof.